The present invention relates to a method and system of measuring an image area of a lithographic printing plate of an offset printing press or the like.
In order to determine the quantity of ink supplied to a printing plate at the time of printing, an image area of the printing plate is conventionally divided into a plurality of zones, and an image area percentage of each zone is measured. Recently, a method is adopted wherein light irradiates the surface of the printing plate, and the image area of each zone is measured in accordance with an electrical signal corresponding to the quantity of light reflected by this zone.
In an image area measuring means disclosed in Japanese Patent Disclosure No. 57-73608, an output signal obtained by reflected light from a blank portion is given to correspond to a 0% dot ratio, and an output signal obtained by reflected light from a solid portion is given to correspond to a 100% dot ratio. By using these ratios as references, intermediate output values are converted to corresponding area percentages, respectively. In practice, a maximum output value for a blank portion is used as a reference for 0% dot ratio, and a minimum output value for the overall area of the printing plate is given as a reference for 100% dot ratio.
However, in the means described above, since the printing plate comprises an aluminum plate roughened by graining, coated with a sensitizer and applied with a gum arabic solution, a surface density becomes nonuniform due to a nonuniform graining, and nonuniform application of the sensitizer or gum arabic. When the maximum and minimum outputs are given to be references for 0% and 100% dot ratios, respectively, an error occurs due to the nonuniform density. It is, therefore, impossible to properly measure the area over the entire surface of the printing plate, resulting in inconvenience.